1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the same.
Note that in this specification, a semiconductor device refers to any device that can function by utilizing semiconductor characteristics, and a semiconductor memory device, an electro-optical device, a semiconductor circuit, an electronic device, and the like are all included in the category of semiconductor devices.
2. Description of the Related Art
Attention has been focused on a technique for forming a transistor using a semiconductor film which is formed on a substrate having an insulating surface. The transistor has been widely used for semiconductor devices such as integrated circuits (ICs) and image display devices (display devices). A silicon-based semiconductor film is known as a semiconductor film applicable to a transistor. As another semiconductor film, an oxide semiconductor film has been attracting attention recently.
For example, a transistor using the following amorphous oxide semiconductor film is disclosed: the amorphous oxide semiconductor film contains indium, gallium, and zinc and has an electron carrier concentration of lower than 1018/cm3 (see Patent Document 1).
Since the oxide semiconductor film has a high electron mobility, the operation speed of a transistor using the oxide semiconductor film is significantly higher than that of a transistor using an amorphous silicon film. There is also an advantage that capital investment can be reduced because part of production equipment for a transistor using an amorphous silicon film can be retrofitted and utilized.
Further, a memory element utilizing the characteristics of a transistor including an oxide semiconductor film is disclosed (see Patent Document 2). The oxide semiconductor film can be deposited by a technique for forming a thin film, such as a sputtering method. A transistor using the oxide semiconductor film can be manufactured in a low-temperature process at 350° C. or lower. Therefore, the transistor using the oxide semiconductor film can be manufactured over another transistor with few limitations, resulting in a reduction in the cell area.